1. Field of the Invention
The present invention relates to a mold which is used for forming optical glass and an optical glass element molded by the mold.
2. Description of the Related Art
In recent years, for example, after molten optical glass gob is supported by molds, it is immediately press-molded, so that optical glass elements such as lenses are manufactured. Since such glass molding is normally performed in high-temperature air, oxidation of the molds extends. When the molds are oxidized, the molding surfaces of the molds become rough, and thus mirror plane properties are deteriorated or the optical glass is fused onto the molding surfaces. As a result, release characteristics are deteriorated, and thus the life of the molds becomes short.
Various efforts are made for lengthening the life of the molds. For example, Japanese Patent Application Laid-Open No. 4-260619 discloses a mold whose molding surface is coated with a film made of chromium metal (Cr) in order to protect the molding surface of a mold base material. Since the mold which is coated with the chromium metal (Cr) thin film has the chromium metal (Cr) film with low hardness, shock resistance and wear resistance with respect to glass are inferior. The release characteristics are not satisfactory in some types of glass to be molded. Further, Japanese Patent Application Laid-Open No. 2-74531 discloses that after a molding surface is coated with a chromium nitride (CrN or Cr2N) film in order to protect the molding surface of a mold base material, the molding surface is subject to heat treatment in an atmosphere including oxygen, and a chromium trioxide film is formed on the top surface. The chromium nitride film is normally composed of a nonstoichiometric compound including Cr and N such as CrN and Cr2N, and thus it is difficult to manufacture a film including only CrN. The chromium nitride film is formed by physical and/or chemical film deposition method(s) such as sputtering, ion plating and CVD. With these film deposition methods, after source materials of chromium nitride are once separated, a chromium nitride compound is formed. For this reason, the composition has defects such that a desired composition cannot be obtained, or the composition is locally dispersed.